Interaction of hyperthyroidism and hindlimb suspension on skeletal myosin heavy chain expression.

We examined the novel interaction of hyperthyroidism and hindlimb suspension on the pattern of myosin heavy chain (MHC) expression (mRNA and protein) in skeletal muscles. Female Sprague-Dawley rats were assigned to four groups: 1) normal control (Con); 2) thyroid hormone treated [150 micrograms 3,5,3'-triiodothyronine (T3). kg-1. day-1] (T3); 3) hindlimb suspension (HS); or 4) T3-treated and HS (T3 + HS). Results show for the first time the novel observation that the combination T3 + HS induces a rapid and sustained, marked (80-90%) downregulation of type I MHC gene expression that is mirrored temporally by concomitant marked upregulation of type IIb MHC gene expression, as evidenced by the de novo synthesis of type IIb MHC protein in the soleus. The fast type IIx MHC isoform showed a differential response among the experimental groups, generally increasing with the separate and combined treatments in both the soleus and vastus intermedius muscles while decreasing in the plantaris muscles. The fast type IIa MHC was the least responsive to suspension of the MHCs and reflected its greatest responsiveness to T3 treatment while also undergoing differential adaptations in slow vs. fast muscle (increases vs. decreases, respectively). These results confirm previous findings that all four adult MHC genes are sensitive to T3 and suspension in a muscle-specific manner. In addition, we show that T3 + HS can interact synergistically to create novel adaptations in MHC expression that could not be observed when each factor was imposed separately.